<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        .box {
            /* width: 500px; */
            height: 300px;
            background-color: #ff0;

            display: flex;
        }

        .a1 {
            width: 200px;
            border: 1px solid #000;

            /* 不收缩 */
            flex-shrink: 0;
        }

        .a2 {
            width: 100%;
            border: 1px solid #000;
        }
    </style>
</head>

<body>


    <div class="box">
        <div class="a1"></div>
        <div class="a2"></div>
    </div>

</body>

</html>